1. Field of the Invention
This invention relates to drilling generally horizontal bores through frangible material beneath surface structure, such as roadways for example.
2. Description of the Related Art
Drilling through frangible material under roadways or other surface structure is usually effected by a percussion tool which progressively drills into the earth. The percussion tool is generally threadingly connected by means of a sub assembly or adapted member to the end of a distal-most section of a drill stem. As the tool progresses into the hole, additional sections of the drill stem are coupled to the end of the last or proximal-most drill stem section in string-like fashion. Each stem of prior art equipment generally includes either a central passage which allows pressurized working fluid to be supplied to the sub-assembly which, in turn, serves to direct the pressurized working fluid to the working chamber of the percussion tool or hammer, or multiple passages to supply working fluid to the working chamber and another passage to provide relatively incompressible "mud" to the proximity of the working area to flush debris away from the working area.
In the former type of arrangements, the air pressure is released from the drill bit inside the hole and forced, together with the cuttings and any other debris in the hole, including water, upward into the annular space between the drill stem and the wall of the hole and out into the atmosphere. In this way, the hole is continually flushed and kept relatively clean to enable the surface being drilled to be broken up by the pneumatic action of a reciprocating bit. If the escape route for the released pressurized fluid should become blocked, such as by water mixed with debris for example, substantial pressures even with compressible fluids may quickly build up underground with the potential to bulge and damage road surfaces for example.
A primary difference with the latter type of prior art arrangement is that pieces of rock or cuttings cut by the drill bit are removed from the hole by drilling fluid or mud. The drilling mud is circulated by mud pumps through the center of the drill pipe, out through holes in the drill bit, and back around the outside wall of the drill pipe. As the mud is forced from the hole, it carries the rock cuttings with it. Again, if the escape route for the pressurized mud should become blocked, the potential for building substantial pressures trapped underground is even greater with the substantially incompressible mud.
Because of the potential environmental hazards that may arise from releasing pressurized fluids underground, such as causing bulging of road surfaces when drilling beneath a roadway, rules and regulations have been, or are now being, promulgated to prohibit such releasing of pressurized fluids in near surface bores. Thus, what is needed is a system whereby pressurized fluid can be circulated therethrough in a closed, fluid-tight arrangement such that no fluids, compressible or incompressible, are released inside a bore being drilled in frangible material that might otherwise create an environmental hazard.